Various types of detectors have been devised in the past wherein a pair of electrodes disposed within a reference chamber and a measurement chamber are employed to measure changes in the level in ionization within the respective chambers which are produced by the presence of smoke, gas or aerosols within the measuring chamber. The presence of smoke or other similar foreign elements within the measuring chamber results in a change in the electric field between the electrodes of such chamber which in turn produces an electric signal. The electric signal produced by changes in the electric field within the measuring chamber is then compared to the electric signal produced by the electrodes in the reference chamber and the resultant signal is subsequently applied to electric instrumentation such as a vacuum tube, field effect transistors or other amplifying means in order to produce a signal for triggering an alarm.
Prior art detectors of the type described above include a number of difficiencies. First, prior art detectors which are highly sensitive are unable to discriminate between an alarm signal produced as a result of the detection of pollutive contaminates which are suspended in the ambient environment, and genuine alarm signals produced by smoke, fire or the like. In some cases, these prior detectors may trigger false alarms as a result of faulty insulation of electrical components or leads within the electrical circuit of the detector that may produce temporary, intermittent short circuits. Similarly, the electronic circuits associated with prior detectors are incapable of differentiating between genuine alarm signals and those signals produced by the effects of the electrical interference, variation in supply voltage, temporarily cutoff in the electrical supply and faulty operation of a detector head. Also, these detectors are prone to producing false alarm signals as a result of chemical or electrochemical corrosion caused by the atmosphere within the measuring chamber.
The present invention overcomes the disadvantages inherent in prior art detectors of the type described above. The detector of the present invention employs means which prevent corrosion of the detector heads whereby to eliminate the production of false alarm signals due to the inadvertant detection of corroded materials within the ionization chambers.
A time delay circuit is provided for preventing premature delivery of an alarm signal produced as a result of a temporary short or open circuit condition in the detector which is unrelated to an alarm condition. Memory circuit means are provided which operate on the electrical alarm signals to temporarily memorize and hold the latter in order to filter out components in such signals which are produced by temporary short circuit conditions or random electrical noise in the detector circuit. According to the present invention, there is provided a reference chamber containing a volume of reference gas therein, and a measurement chamber communicating with the ambient atmosphere for containing a volume of air characteristic of the surrounding atmosphere. A radioactive source disposed adjacent each of the chambers ionizes the contents in each of the chambers. Each of the chambers is formed by a varnished enclosure structure. A pair of electrodes are coupled with each of the chambers and are polarized by dc current, each pair of which electrodes function, in effect, as a pair of capacitors. A change in the degree of ionization within either of the chambers results in a change in the electric field between the electrodes of the corresponding chambers. Normally, the electric field, and thus the electric current flow, between the electrodes in the reference chamber remains essentially constant, while the electric field, and resulting current flowing between the electrodes in the measuring chamber changes in relation to the chemical composition of the air in the ambient atmosphere which is allowed to circulate through the measuring chamber. Combustible products such as smoke or the like produced by a fire in the vacinity of the detector drifting into the measuring chamber are ionized and result in a change in the magnitude of the electric field between electrodes in the measuring chamber, thereby giving rise to a change in current flow between such electrodes. The change in current derived from the measuring chamber is amplified by an electronic circuit and is employed to produce a visual or audible alarm.
A velocimetric circuit operably coupled with each of the ionization chambers functions to adjust the impedance of the detector, thereby adjusting the quiescent operating point of the detector, and functions to compare the rate of increase in the magnitude of the electric signals derived from the electrodes of the respective ionization chambers in order to eliminate those components of such signals produced as a result of electrical interference or defective insulation of portions of the circuit in the detector.
Memorization circuit means are provided to analyze and differentiate between electrical signals of various types corresponding to different stimuli such as flame, temperature variation, etc. A first visual or audible annunciator is responsive to an alarm signal produced by the detector circuit to announce the existence of an alarm condition, while a second annunciator is responsive to the production of alarm signals in the event that the first mentioned annunciator is rendered inoperative.